This invention relates to the field of waveguide beyond cutoff couplers. Several difficulties have been experienced with the use of known waveguide beyond cutoff couplers in a cryogenic environment. These difficulties include fragile components, complex machine work for mechanical guidance of the control structure, such as slotting the waveguide on the side, inserting teflon screws and control with a rod outside the apparatus, difficulty in sealing the connection of the control to the cavity structure from cryogenic fluids and coupler rod shrinkage when operating in cryogenic fluids.